...without added stripes.
Myles has got the mixer nozzle (see here) working!
The two filaments left and right were mixed to print the component in the middle. (Myles says sorry for the lousy iPod photos.) The mixer is now active - it has a small DC motor turning a stirrer in the brass hot end to churn the melt up, and that seems to do the trick.
Next stop - full five-colour operation!